Measuring circuit with time-varied electric filter



June 14, 1960 J. A. K. RICHARDS 2,9

MEASURING CIRCUIT WITH TIME-VARIED ELECTRIC FILTER Filed Dec. 19, 1955 SECONDS INVENTOR. J. 4. K. Richards RECORDING VOLTMETER ATTORNEY TIME oi24se1a SECONDS United States Patent MEASURING. CIRCUIT WITH TIMEQVARIEDJ ELECTRIC FILTER:

James A. K. Richards van'xNuys, Califi, assignor to Bendix Aviation Corporation, NorthHoiiywood, Calih, a corporation. ofDelaware FiledDe'c. 19, 1955,3ei. No. 553,774

3 Claims; (Cl. 324-140) This invention relates to electrical filters for sepa-- rating currents of different frequencies, and is particular, ly useful in, although not limited to, low-passfilters-for; suppressing alternating currents and passing direct cur-.- rents.

It is well recognized that following application of 0 an input potential to a conventional filter there isa transient period before the output potential reaches a 1 steady state. In many uses of filters, this delay in reach: ing a steady state condition is of no importance, but there are.;situations -in which it is objectionable. One such situation. is, thatdn which the-value ofthe :direct component, oi-a composite potential consisting of DC. and1A.C..components is to bemeasured at a given in-j'- stant. The. measuring apparatus, including. a filter to suppress the,A.C.,component,--is connectedto the ClI'Cl1lL3Q carrying thetcomposite potential at the instantthe measurement is desired, but with conventional filters there is an-.. objectionable ,time lag between the. connection andzthe. indication by. the measuring. apparatus of ..the true value of the.;D.C.. component.-

An .object. of, the. present, invention is to. reduce the time lag of an, electrical filtrwithout correspondingly, impairing, its suppression; characteristics for. undesired, frequencies.

Briefly, this object is attained by varying .thdimpedance offlone or, more of thefilter-elementsas a function'of time duringwhatwould otherwise be the..tran-.. sient period of thefilter,

Other more specific objects and ,featuresoftheflinvenu tion will appear from the following detailed description; withrefe'rence to the drawing, in which Fig l is a diagram of a simple' circuit incorporating, the invention- Fig. 2 is a schematic circuit offa conventional filter having fixed constants.

Fig. 3 is a schematic circuit. of the variable filter in the system of Fig.1.

Fig. 4 isa graph showing the characteristics of a'prior 1 art'filterin accordance with Fig. 2.

Fig. 5 is a graphshowing the characteristics of the filtenof the present invention in accordance with Fig. 3.

Fig; 6 is a graph showing the ramp characteristics of the filters of Figs. 2 and 3, respectively. I

Fig. 7 is a diagram of a voltage checking system'using the invention,

Referring to Fig. 1, there is shown a circuit incorporating the, invention for measuring at any given instant the: potential'of the D.C.. component of a composite potential applied between ground and an'input terminal" 10.1 This potential is identified, as e (t), since it varies with time. The essential componentsof the circut are a switch 11,a filter 12 consisting of a series, rheostat 13" and a shunt condenser 14, and a voltmeter 15. Actuation of the push-button switch 11 energizes avmotor 16. whichidrives the ;rheostat 13 through ,a slip clutch .18.... Asfshown, thevrheostat 13 comprises an arcuate resist-. ance element and a rotary slide contact connected to the ice.

terminalp10. In the-normal starting positionv shown,- the rheostat-circuitis open Upon closure of the switch 11, the movable contact first connectsithe terminaltlotf directly to the condenser 14 and then moves over the resistance .elementeto introduce resistance between terminal 10 and condenser 14 at a rate dependent upon the value of the resistance element and-the speed of the motor 16. As will appear later, the resistancepreferably increases linearly with time. A linear rate'can be approximated by. employing a fast-starting, constant-.-

speed motor and/or tapering the rheostat'to compensate for the acceleration rate of the motor. Alternatively, the

motor can be running, and the rheostat slidecanbe'connected thereto by a clutchfatthe instant'of closure of the switch 11. The essence of the invention is a filter circuit as shown in Fig. 3, in which the resistance element 13 is varied from zero resistance, at the instant of starting, to a predetermined value R(t) within a time span that is determined by the characteristics of the potential to be measured, particularly with reference to the lowest frequency component to be rejected.

If, as in Fig. 2, the resistance element 13 were'fixed, and it and the condenser 14 were large so as to substantially suppress low-frequency alternating current, the condenser 14 would have a substantial charging time. Hence, the output potential e (1) resulting from application to the input terminals of a potential e (z) having a slowly varying D.C. compotent, would rise slowly as the condenser 14 charged, and would require a considerable transient period to reach a steady state value indicative of the value of the DC. input component. This conditionis illustrated in the graph of Fig: 4,.in which several? seconds are required following application of the input potentialto the filter for the output potential e '(t).- to.

rise to the value of the D.C.'component of the input po-v tential e (t). Obviously, with the prior art filter of Fig. 2, readings cannot be taken in rapid succession.

The transient period is reduced in accordance with the invention by varying the resistance of the rheostat- 13 from an initialvalue of 0 (at the time of application of the input potential e (t), toits full value in a time much less than the transient period of V the circuit of Fig. 2. The initial low value of the rheostat 13 permits the condenser 14 to charge almost instantly to thefull However, the rheostat rises to its full value within 'a potentialtof the DC. component of the input potential: 45 'e (t), but provides little filtering of the AC. component,

much shorter time period than the-normaltransient,

period of the filter having a fixed. resistor, and the out putpotential e 0) reaches a steady statecondlnon much-- sooner, as shown in Fig. 5.

It-wil'l be, observed from Fig. 5 that the potential e (t)- rises almost vertically to the value of the input .po:

tential e (t) at that instant, whereas the time required 7 for the rheostat 13 to attain its full value, in this particular instance, is two seconds. Until the product of RC'has reached a value, equal to that of the passive filter (Fig. 2),. filtering of the A.C. component of the input potential e (t) will not be-as good as that of Fig. 2.

Nevertheless, since the delay is substantially less than with a fixed filter, as shown in Fig. ,4, that value can be reached in much less time than five seconds-thus permitting better filter action.

Fig. 5 is illustrative of the optimum results obtainable when the sourceof the input potential e 1) hasnegligible 1 internal impedance.

In effect, such internal impedance constitutes a fixed impedance in series with thevariable filter resistor 13 and slightly delays the rise of the out pu t potential e 0). When the internal impedance; of the source is appreciable, it is desirable to reduce, 11162 size ofthe capacitance 14v and correspondingly. increaseathe maximum resistance of the resistor 13. If the product of the resistance and the capacity is maintained constant, the A.C. filtering effect is maintained and the generatorto-condenser initial time constant reduced.

For purpose of analysis, assuming that the resistance of thevariable resistor 13 rises linearly, the output potential 2 (1) of the filter shown in Fig. 3 is represented by the following equation: I

. 1 a t) 1 i h tsmi i For a ramp function i G t i more Comparedto the response of the circuit of Fig. 2,

the active, filter 'of Fig. 3 suppresses the ramp in a ratio p 1 RC while the standard circuit (Fig. '2) only delays the ramp but does not suppress it. Fig. 6 shows the difference.

The response to sine waves is similar and may be seen by comparing impedance functions. equation .for the circuit of Fig. 2 is:

1 5144.030 and "for that of Fig. 3 is:

1 +jw'RCt If the product of RC of Fig. 3 is large compared of 'Fig. 2, the suppression of A.C.by the-former equals that of the latter a timeequal to the ratio of the products I in a practical circuit, the voltage source impedance must be finite instead of zero, as -h'as been assumed.

The actual response to a step function would be modified. The timeconstant is then the product of the generator internal resistance and the capacity of the filter. If the former 'is low, it' may be ignored. If not, the filter is still superior, since the capacity rnay be reduced to attain a shorttime constant, and the'filter resistance coefiicientincreased to maintain the product constant.

As an 'example,assume an input source having an approximate potential of one volt' an-d an; internal resistance of 100 ohms. When such a source is connected The impedance to that a to a prior art filter, as shown in Fig. 2, having a resistance of 10 ohms and a capacityof lilf farads, about five seconds are required for the output potential e f to attain the value ofjthe D.C."coi1iponent ofrthe" input I tential (7 f. 'With the same source, but a filterin accordance with 4 the first cycle ofthe rheostat 13), it is immaterial whether the latter continues to run. If it isdesired to take continuous readings, the rheostat is stopped when it reaches its position of maximum value, as by providing a stop pin 13a for blocking the movable contact, the slip clutch 18 permitting such stoppage.

A valuable use of the variable filter of the invention is in test equipment for the rapid sequential measurement of the DC potentials of a plurality of circuits. "Such a test equipment for measuring the potentials on a plurality of terminals 20 is shown in Fig. 7.

The terminals 20 are successively connected by a rotary switch 21 to a common lead 22connected to the movable contact of the rheostat 13. The leading end of the rheostat resistance element is periodically momentarily connected by a rotary switch 23 to a recording voltmeter 24, as the rheostat reaches its maximum resistance during each rotation. Both the rheostat 13 and rotary switch 23 are driven continuously by the motor 16. The rotary switch 21 is driven, as by gears 25 and 26, at a lesser speed such that a diiferent one of the terminals 20 is connected to the rheostat 13 during each successive cycle thereof. Since the use of the variable filter rheostat 13 greatly reduces the time required for each measurement, the potentials on a large number of terminals can be measured in rapid succession in a much shorter time than if a conventional filter were used.

Two modes of operation of the system of Fig. 7 are possible, depending upon the phasing between the switch 21 and the rheostat 13. Thus, they may be so phased that the switch 21 connects a new terminal 20 to the line 22 while the rheostat is in open position; the rheostat then 'completesthe circuit to the filter. On the other hand, the phasing can be such that the movable contact of the rheostat contacts the resistance element just before the switch 21 connects a new terminal 20 to the line 22; the switch 21 then completes the circuit to the filter.

' 6 Although for the purpose of explaining the invention a particular embodiment thereof has been shown and'described, obvious modifications will occur to a person skilled in the art, and I do not desire to be limited to the exact details shown and described.

I claim:

i 1. An electrical filtering circuit comprising: input terminals and output terminals; a shunt impedance element connected across said output terminals; a variable series-impedance element variable between low and high impedances, respectively, and comprising a fixed impedance elernent and a tap movable onto one end of and thereafter along said fixed impedance element; means connecting said tap and said one end to said input and output terminals, respectively; motor means for continuously moving said tap first onto said one end of said fixed element and then therealong, whereby said series element is connected between said input and output terminals while in its low impedance condition, and

theimpedance of said series element is thereafter increased at a predetermined time rate.

2. An electrical filtering apparatus comprising: a plur ality of input terminals and a pair of output terminals; a

' shunt impedanceelement connected across said output terminals; a variable series impedance element variable between low and high impedances, respectively; motor means in driving relation to saidvariable impedance ele ment for varying it between low and high impedances at a predetermined time rate; switching means for selectively Figf' 3, havingfa' resistance'increasing from zero at the rate of 10 ohms per second and a rcapacity' 'of 10* farads,- the output potential 3 0) will approximate the value of the input potential 13 (1) in about .001 second,

and will be very close-to the value of the DC. component'thereof in about two seconds.

If it is desired to obtainonly a single reading (within connecting said series element between any one .of said in put terminals. and one of said output terminals, said switching means comprising a plurality of input contacts connected to said respective input terminals and a common contact sequentially connecting to diflierent inputtcontacts, and means driven by said motor for actuating said switching means for successively connecting said common Glltact to'different input contacts during successive low impedance conditions of said variable impedance element.

3. Apparatus according to claim 2 including electrical indicating means and switching means actuated by said motor means for connecting said output terminals to said indicating means during successive high impedance con- 5 ditions of said variable impedance element.

References Cited in the file of this patent UNITED STATES PATENTS 2,435,195 Bomberger m1 Feb. 3, 194:;

6 Gibbs et al May 10, 1949 Marlowe et a1 May 24, 1949 Philpott June 21, 1949 Dieke Aug. 30, 1949 Wilmotte May 16, 1950 Heller Oct. 10, 1950 Hippie et a1 Nov. 20, 1951 Ragland June 26, 1956 Bernet Dec. 18, 1956 Berry Oct. 14, 1958 

